GRK, you are 100% wrong on this one. ;D The radius you are talking about does indeed exist, and it is different for different types of unit (infantry, chopper) but it doesn't really have a recognised name and is definitely not what we are talking about.
AI are not supposed to be able to go through/see through solid walls and unenterable buildings but at the margin they can. This is one of the constraints of OFP and it is why it is impossible, for example, to create a really good urban combat mission. The AI can sometimes shoot through walls and you can't, it's that simple.
Now, turning to the problem in hand, my belief is that the game does have something in it to help prevent bad problems with placement radii. However it is not infallibale and manifestly doesn't cover the water problem. The best solution is extensive playtesting.
In this particular example (Ural can't get through tree) its not really a placement radius question at all. The Ural probably could have got to the waypoint without driving through the tree, but AI drivers are notoriously poor. This is fundamentally a driving question, not a waypoint placement radius question. There are many secrets to improving AI driving but one of the less obvious is to have no objects (tents etc) close to the side of the road. A tent too close to the road may (not usually, but sometimes) cause havoc to AI driving half a mile away.